1. Field of the Invention
The invention relates the prevention of radiation injury to tissues expressing the protein sodium/iodide symporter (NIS) and especially to radiation injury to the lacrimal sac and nasolacrimal duct in cancer patients being treated with high dose radioactive iodine.
2. Description of the Related Art
The sodium-iodide symporter is an integral membrane protein that resides in the basolateral membrane of epithelial cells located in such organs as the thyroid. The sodium-iodide symporter cannot distinguish between normal and radioactive iodide, thus providing a useful exploit for diagnosis and treatment of certain thyroid disease. Small amounts of radioactive iodine (I123) injected into patients are rapidly concentrated in the thyroid, providing a means to image the thyroid for detection of tumors and other abnormalities. Administration of radioiodine (I131) is widely used for treatment of hyperthyroidism and some types of thyroid cancer; in this case the radioactivity is concentrated rather precisely in the tissue requiring destruction.
The sodium-iodide symporter simultaneously transports both Na+ and I− ions from extracellular fluid (i.e., blood) into the thyroid epithelial cell. This process is an example of secondary active transport. Energy is provided by the electrochemical gradient of sodium across the cell membrane; the low intracellular concentration of sodium is maintained by sodium pumps. Although, the sodium-iodide symporter is most highly expressed in thyroid epithelial cells, lower levels of expression can be detected in mammary gland, salivary gland, stomach, and colon tissue. See U.S. Pat. No. 6,803,199, columns 1 and 2.
Ophthalmic complications following I131 radiation therapy have been observed in a significant percentage of patients being treated for thyroid cancer. Symptoms such as ocular dryness, epiphora (watering of the eyes due to obstruction of the lacrimal passages), dry mouth (xerostomia), and nasolacrimal duct obstruction (NLDO) have been observed. Moos et al., J. Clin. Endocrinol. Metab. 2002 December; 87(12): 5817-20 and Burns et al., Opthal. Plast. Reconstr. Surg. 2004 March; 20 (2):126-9.
It is believed that the cases of NLDO observed in some patients receiving high dose radioiodine treatment is due to the concentration of radioactivity by the sodium-iodide symporter. High levels of radioactivity concentrated by NIS in a relatively small area in the lacrimal sac and nasolacrimal duct causes fibrosis which results in blockage in the lacrimal duct and nasolacrimal sac. It is also believed that some cases of dry mouth observed in patients being treated with high does radioiodine for head and neck cancers is due to accumulation of radioactivity in the salivary glands leading to fibrosis which blocks release of the salivary fluids.
The incidence of newly diagnosed head and neck cancers (excluding skin cancers) in the U.S. is estimated at more than 50,000 cases annually. The most common type of cancer in the head and neck is squamous cell carcinoma, which arises in the cells that line the inside of the nose, mouth, and throat. Other less common types of head and neck cancers include salivary gland tumors, lymphomas and sarcomas. In addition to head and neck cancers, there are over 15,000 new cases of thyroid cancer each year in the United States.
Thyroid cancer is typically treated with surgery followed by radiation therapy. The three main types of treatment for managing head and neck cancer are radiation therapy, surgery, and chemotherapy with the primary treatment being radiation therapy or surgery, or both combined.
In addition to being used in the treatment of head and neck and thyroid cancers, radioactive iodine (I131) is widely used to treat hyperthyroidism. Hyperthyroidism results from excess quantities of thyroid hormone within the body. Rather than being classified as a specific disease, it is classified as a syndrome that describes the characteristics resulting from this condition. The causes of hyperthyroidism include Graves' disease; tumors of the thyroid gland, pituitary gland, testes or ovaries; inflammation of the thyroid from a viral infection or other inflammation; ingestion of excessive amounts of thyroid hormone; and ingestion of excessive iodine. Graves' disease accounts for 85% of all cases of hyperthyroidism. The incidence is 1 out of 1,000 people.
Finally, doctors are increasingly using radioiodine to treat breast cancer. Radioactive I131 is given either orally or by injection. Using I131, the clinician is able to selectively target the cancerous breast tissue as opposed to normal breast tissue.
In the United States each year, as many as 100,000 people may be treated with high dose I131. This number is expected to increase as the use of I131 to treat breast cancer becomes more common. With the increased use of high dose radioiodine, it is expected that the incidence of NLDO will increase as well.
The traditional procedure most often relied on for relief of NLDO is incisional dacryocystorhinostomy (DCR). This procedure has a number of drawbacks: recovery time is significant, an incisional scar may develop due to invasive procedures, there is potential for excess bleeding, the procedure must be done under anesthesia (usually general anesthesia), and the costs associated with the surgical procedure are not trivial. The DCR procedure has a high success rate in patients suffering from NLDO caused by other than high dose radioiodine; however, in patients where the nasolacrimal duct is obstructed as a result of receiving high dose radioiodine, the DCR procedure does not work very well.
Currently, there is not available a safe and effective method for preventing the fibrosis that can occur in the nasolacrimal duct area due to the administration of high dose radioiodine. A method that prevents or reduces the formation of fibrosis in the nasolacrimal duct rather than treats fibrosis after it forms, is highly desirable. The method of the invention described herein avoids all of the drawbacks associated with DCR. It is a cost effective, safe, non-invasive method that utilizes topical application of perchlorate anion in ophthalmic solutions, ophthalmic creams, or gels to block the ability of the sodium-iodide symporter to concentrate radioactivity. The method of the invention is so safe and effective and cost effective that it should be the standard of care for every patient receiving high dose I131 therapy for treatment of cancer and especially for treatment of head and neck, thyroid, and breast cancers.